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The battery within a UPS must be sized to provide a specified runtime (also known as battery autonomy), in minutes or hours, at a specific load.

Load-specific factors are incorporated into a general battery sizing algorithm that will also take into account a number of other issues relating to ambient temperature and the inverter and system efficiencies of the UPS being considered.

A battery set may comprise of a single or multiple battery strings connected in parallel. A battery string consists of one or more battery blocks connected in series, with the ampere-hours (Ah) rating of the blocks used to define the total Ah of the battery set.

Whilst battery manufacturers provide discharge curves and tables to assist selection, UPS manufacturers generally have bespoke battery sizing programs designed around their own UPS characteristics.

Their battery packs also tend to be somewhat uniform, based around battery ranges and their source of battery cabinets or stands.

For any given load, the runtime period must be sufficient to keep the loads running, allow the start-up of standby generators (both automatic and manual if necessary) and sufficient time for the completion of automatic load shedding or shutdown routines.


Factors To Consider When Sizing A UPS Battery:

  • Site power history, and in particular, the duration of previous mains power supply failures
  • Critical load sizes
  • If there is a maintained standby generator on site
  • The length of time required for load shedding and/or an orderly shutdown
  • The available environment, for example, space, temperature and humidity
  • Cost versus space requirements for longer runtime systems.


Comparing battery autonomy between different UPS manufacturers is often tricky. The most effective method is to ensure all runtimes are quoted using the same load Watt ratings and load power factors.

For more information, or support sizing your UPS battery system, contact our sales team.

Alternatively, we've developed an online UPS Configurator to help you find the right UPS for your load.


Further Reading:



Lifespan of ups batteries

Batteries are essential for UPS correct functioning. In this whitepaper we explain the main important factor that affect their duration and we provide some recommendations for optimizing the UPS + batteries system, in order to maximize its lifespan and general availability.